The moss wall cave was proven as a glitch, but you don't need codes to do it.
Step 1: Go to Freezeezy Peaks. Naturally.
Step 2: Start flying directly into the moss wall.
Step 3: Point Banjo and Kazooie directly upwards. They should start falling downwards if you've done this correctly.
Step 4: Go mental on the A button! Keep tapping it, and pointing the pair upwards. Look behind the wall... and BLAM! Instant cave.
As I've said, there is no code for it, as it isn't hacking, but it has been proven as a glitch. Sorry.